procedurally generated mold (fungus) using voronoi texture?
Your technique seems to be the most adequate for the illustrated effect, just needs some tweaking.

All you have to do is run your Voronoi texture through two Color Ramps, one for transparency and one for color.

Add two stops to each so you can control progression smoothly.

You can control "cluster size" from a black to white color ramp connected to the shader mix node.



You can also control color variation with a green to white ramp independently, making cells more or less greenish.



For cell spacing you have to adjust the Voronoi texture size, on the texture node itself. If you increase the size the cells will be bigger and more spread out, you will then have to manually adjust the two other color ramps to make each cell "tighter", otherwise it will just increase size, without creating bigger gaps.



If you want better control over the color of your mold, instead of directly using a Color Ramp from green to white for color, use a mask color ramp from black to white conected to a Color Mix node Factor socket, and you can then use any other textures for actual diffuse.